Add helper copy_file_range() which:
- avoid copy data into userspace
- may support reflink on xfs etc

Signed-off-by: Jiang Liu <[email protected]>

- improve copy_file_range when target os is not linux
- add more comprehensive tests

Signed-off-by: Yadong Ding <[email protected]>

Implement CasManager to support chunk dedup at runtime.
The manager provides to major interfaces:
- add chunk data to the CAS database
- check whether a chunk exists in CAS database and copy it to blob file
  by copy_file_range() if the chunk exists.

Signed-off-by: Jiang Liu <[email protected]>

- Changed `delete_blobs` method in `CasDb` to take an immutable reference (`&self`) instead of a mutable reference (`&mut self`).
- Updated `dedup_chunk` method in `CasMgr` to correctly handle the deletion of non-existent blob files from both the file descriptor cache and the database.
- Implemented the `gc` (garbage collection) method in `CasMgr` to identify and remove blobs that no longer exist on the filesystem, ensuring the database and cache remain consistent.

Signed-off-by: Yadong Ding <[email protected]>

Enable chunk deduplication for file cache. It works in this way:
- When a chunk is not in blob cache file yet, inquery CAS database
  whether other blob data files have the required chunk. If there's
  duplicated data chunk in other data files, copy the chunk data
  into current blob cache file by using copy_file_range().
- After downloading a data chunk from remote, save file/offset/chunk-id
  into CAS database, so it can be reused later.

Co-authored-by: Jiang Liu <[email protected]>
Co-authored-by: Yading Ding <[email protected]>
Signed-off-by: Yadong Ding <[email protected]>

Add documentation for cas.

Signed-off-by: Jiang Liu <[email protected]>

Add smoking test case for cas and chunk dedup.

Signed-off-by: Yadong Ding <[email protected]>